Conventionally, a technical development for enabling increasing the recording capacity of a piece of an optical disc has been carried out. As one among those, which is a technique of doubling the recording capacity per an optical disc, a double-layer disc that has multiplied two recording layers has been realized.
The interlayer movement is performed by after making the focus control once opened, applying to a focus actuator such an acceleration pulse that makes the objective lens move toward the object layer, applying a deceleration pulse thereto in the vicinity of the object layer, and again closing the focus control. The width of the deceleration pulse is determined by measuring the time which was required for moving over a predetermined distance from the layer before movement to the vicinity of the object layer. Besides, since the inter-layer distance varies depending on the disc, it is obtained on the basis of the focus error signal which was obtained when the objective lens is previously moved upward and downward (for example, refer to patent reference 1).
By the way, a development of a multi-layer disc having two or more layers has been advancing. For example, it is a multi-layer disc of four or six layers.
FIG. 18 shows an example of a four-layer disc. In FIG. 18, a case is shown where the interlayer distances between respective layers are 15 μm and the laser light which is collimated by the objective lens comes in the focused state in L0 layer. In this state, the laser light which is partially reflected at the intermediate L1 layer comes in the focused state in L2 layer, being superposed on the reflected light from the L0 layer. Therefore, the quality of the reproduced signal is deteriorated. Accordingly, a multi-layer optical disc having disc specifications that include different interlayer distances as shown in FIG. 2 is proposed.
In addition, in order to increase the recording capacity per disc, wavelength shortening of a light source and increasing of Numerical Aperture (NA) of an objective lens are attempted. For example, an optical head having a wavelength of 405 nm and an NA of 0.85 is in practical use. Here, when the thickness of a cover layer of the disc is deviated, a spherical aberration occurs, which spherical aberration increases with an increase in Numerical Aperture of the objective lens. Accordingly, since there arises distortion in the focus error signal influenced by the spherical aberration in such a high density disc, the optical head is provided with a correction mechanism for correcting the spherical aberration which should be generated.
By the way, a focus actuator is generally designed in a voice coil type so as to have a high response speed, while the spherical aberration correcting mechanism which employs a liquid crystal or a stepping motor is designed to have a response speed that is about ten times slow with relative to the focus actuator. Accordingly, it is configured such that, when performing an interlayer movement from an information layer to another information layer, the spherical aberration correcting value is switched to the optimum value for the layer of movement destination in the layer of movement source, and thereafter, the focus actuator is driven to make the focal point position of laser light move to the layer of movement destination (for example, refer to patent document 2).    Patent Document 1: Japanese patent published application No. Hei. 10-143872 (page 1, FIG. 1, and the like)    Patent Document 2: Japanese patent published application 2003-22545 (page 1, FIG. 1, and the like)